Transit diversity wireless communication

ABSTRACT

A method of transmitting data from a transmitter ( 13,24 ) to a remote receiver ( 14,28 ) using transmit diversity wireless communication, the transmitter ( 13,24 ) comprising three or more transmit antenna elements ( 1,2,3,4 ). The data is encoded in symbol blocks ( 12 ), the symbols (s 1 ,s 2 ,s 3 ,s 4 ) of a block ( 12 ) being permuted within respective sub-sets of symbols and permuted symbols between the transmit antenna elements (1,2,3,4) over time with respective replications and-complex conjugations and/or negations. At least one of the sub-sets of said transmit antenna elements ( 1,2,3,4 ). The signals transmitted over at least one of the transmit antenna elements ( 1,2,3,4 ) are modified as a function of channel information at least approxiamately related to the channel transfer function (h 1 ,h 2 ,h 3  and h 4 ) of the transmitted signals, and the sub-sets of symbols and permuted symbols are permuted over time between said sub-sets of transmit antenna elements, so that the received signal is detectable at the receiver ( 14,28 ) using an orthogonal detection matrix scheme.

FIELD OF THE INVENTION

[0001] This invention relates to transmission of data by transmitdiversity wireless communication.

BACKGROUND OF THE INVENTION

[0002] Wireless communication systems are assuming ever-increasingimportance for the transmission of data, which is to be understood inits largest sense as covering speech or other sounds and images, forexample, as well as abstract digital signals.

[0003] Currently proposed standards for wireless communication systemsbetween a stationary base station and a number of remote (mobile orimmobile) stations include the 3GPP (3^(rd) generation PartnershipProject) and 3GPP2 standards, which use Frequency Division Duplex(‘FDD’) or Time Division Duplex (‘TOD’) and Code Division MultipleAccess (‘CDMA’). The HIPERLAN and HIPERLAN2 local area network standardsof the European Telecommunications Standards Institute (‘ETSI’), useTime Division Duplex (‘TDD’) and Orthogonal Frequency Division Multiplex(‘OFDM’). The International Telecommunications Union (‘ITU’) IMT-2000standards also use various multiplex techniques of these kinds. Thepresent invention is applicable to systems of these kinds and otherwireless communication systems.

[0004] In order to improve the communication capacity of the systemswhile reducing the sensitivity of the systems to noise and interferenceand limiting the power of the transmissions, various techniques are usedseparately or in combination, including space diversity, where the samedata is transmitted over different physical paths interleaved in time,in particular over different transmit and/or receive antenna elements,and frequency spreading where the same data is spread over differentchannels distinguished by their sub-carrier frequency.

[0005] At the receiver, the detection of the symbols is performedutilising knowledge of the complex channel attenuation and phase shifts:the Channel State Information (‘CSI’). The Channel State Information isobtained at the receiver by measuring the value of pilot signalstransmitted together with the data from the transmitter. The knowledgeof the channel enables the received signals to be processed jointlyaccording to the Maximum Ratio Combining technique, in which thereceived signal is multiplied by the Hermitian transpose of theestimated channel transfer matrix.

[0006] Two broad ways of managing the transmit diversity have beencategorised as ‘closed loop’ and ‘open loop’.

[0007] Two closed loop methods are described in the paper entitled“Transmit adaptive array without user-specific pilot for 3G CDMA” by B.Raghothaman et al., that appeared in the IEEE Transactions 2000. In thesystems described in this paper, the signals transmitted over thedifferent transmit antenna elements of the base station are weightedaccording to relative weights calculated at the receiver from ChannelState Information and retransmitted to the transmitter. In one systemreferred to, pilots specific to each user are transmitted in addition tothe pilots for each transmit antenna element that are common to allusers, which penalises the communication capacity of the system. Inanother system disclosed in the paper, user-specific pilots are avoidedby re-modulating the detected signals using the measured Channel StateInformation and the calculated weights and using the re-modulatedsignals to correct errors in feedback; this imposes a heavycomputational load on the receiver and the result is only reliable ifthe channel state estimation is sufficiently correlated with the actualchannel state to avoid a high detection error rate.

[0008] In pure ‘open loop’ methods, no Channel State Information is fedback to the transmitter. In such systems, the transmitter comprises aplurality of transmit antenna elements; the data is encoded in symbolblocks, the symbols of a block being permuted between the transmitantenna elements over time with respective replications and complexconjugations and/or negations. The complexity of the receiver depends onthe properties of the matrix that defines this space-time block code; inparticular detection is performed with a low cost in terms of simplicityof the receiver computations if this matrix is an orthogonal one.

[0009] An open loop system using an orthogonal detection matrix isdescribed in International Patent Application Publication No WO 99/14871Alamouti. In this system, the symbols of a block transmitted arepermuted between the transmit antenna elements over time with respectivereplications and complex conjugations and/or negations according to ascheme, known as the ‘Alamouti code’, such that the received signal isdetectable at the receiver using an orthogonal detection matrix scheme.

[0010] The performance of the code is mainly based on the diversityorder of the code. This diversity order characterizes the number oftransmit and receive antennas which is actually seen by the code. For agiven number of receive antenna elements, the more transmit antennaelements are used the more improvement is obtained in terms of fadingand interference is obtained. However, the paper entitled “Space-TimeBlock Codes from Orthogonal Designs” by V. Tarokh et al. that appearedin IEEE Transactions on IT, vol. 45, July 1999, states that anorthogonal detection code matrix can not be used if the transmittercomprises more than two transmit antenna elements with full diversitywithout sacrificing the coding rate, that is to say the useful data ratefor the user. They propose coding rates of ½ for three to eight transmitantenna elements or ¾ for three or four transmit antenna elements.

[0011] Patent specification WO 00/51265, Whinnett et al., assigned toMotorola, describes another transmit diversity system, in which coderate is maintained for arrays of more than two transmit antenna elementsbut at the expense of sub-optimal transmit diversity.

[0012] Another transmit diversity scheme (ABBA code) is described formore than two transmit antenna elements in the paper entitled “MinimalNon-Orthogonality Rate 1 Space-time Block Code for 3+Tx Antennas” by 0.Tirkkonen et al. IEEE 6^(th) Int. Symp. On Spread-Spectrum Tech. &Appli., NJIT, pp. 429-432, September 2000. This coding rate 1 scheme isderived from the permutation of two Alamouti codes as described by thecode matrix $\quad\begin{bmatrix}A & B \\B & A\end{bmatrix}$

[0013] It is stated that the ABEA code provides full spatial diversityto the detriment of the orthogonality of the detection matrix, whichimplies that the computational cost of the detection step is increasedcompared to an orthogonal scheme. In addition the performance of theideal code is not fully achieved by the ABBA code due to theinterference terms of the detection matrix.

[0014] Other compromises are proposed in a paper presented by H.Jafarkhani to the IEEE Wireless Communications and Networking Conferencein September 2000 with non-orthogonal detection matrices that are statednot to achieve simultaneously the optimum diversity and transmissionrate, two encoding schemes proposed being of the kind described by thecode matrices $\begin{bmatrix}A & B \\B^{*} & {- A^{*}}\end{bmatrix}\quad {{{and}\quad\begin{bmatrix}A & B \\{- B^{*}} & A^{*}\end{bmatrix}}.}$

[0015] Yet another compromise is described in the paper “A randomisationtechnique for non-orthogonal space-time code blocks” by A Hottinen etal. appearing in IEEE VTC 2001. However, this system still does notemploy an orthogonal detection matrix with full diversity for more thantwo transmit antenna elements.

[0016] Still another compromise is described in the paper “A space-timecoding approach for systems employing four transmit antennas” by C. B.Papadias et al. presented at an IEEE conference in 2001 and thatproposes an encoding scheme of the kind $\begin{bmatrix}b_{1} & b_{2}^{*} & b_{3} & b_{4}^{*} \\b_{2} & {- b_{1}^{*}} & {- b_{4}} & b_{3}^{*} \\b_{3} & b_{4}^{*} & {- b_{1}} & {- b_{2}^{*}} \\b_{4} & {- b_{3}^{*}} & b_{2} & {- b_{1}^{*}}\end{bmatrix}.$

[0017] This scheme also uses a non-orthogonal detection matrix that doesnot achieve simultaneously the optimum diversity and transmission rate.

SUMMARY OF THE INVENTION

[0018] The present invention provides a method of transmitting data froma transmitter to a remote receiver using transmit diversity wirelesscommunication and a system, a transmitter and a receiver as claimed inthe accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is a schematic diagram of a system for transmitting data bytransmit diversity wireless communication in accordance with anembodiment of the invention,

[0020]FIG. 2 is a schematic diagram of a system in accordance with FIG.1 applied to a time division duplex (TDD), orthogonal frequency divisionmultiplex (OFDM) system, and

[0021]FIG. 3 is a schematic diagram of a system in accordance with FIG.1 applied to a frequency division duplex (FDD), code division multipleaccess (CDMA) system

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022]FIG. 1 shows a first embodiment of a system for transmitting databy a transmit diversity wireless communication network, the systemcomprising a first station that will be described as the transmitterside (with primary reference to its transmission function) and a secondstation that will be described as the receiver side (with primaryreference to its reception function). In the present case, the firststation and the second station are both capable of both transmission andreception and, moreover, the same antenna elements are used both fortransmission and reception in the preferred embodiment of the invention.

[0023] The transmitter side comprises four transmit antenna elements, 1,2, 3, 4. The receiver side of the system comprises an array 5 of Mreceive antenna elements. The number of antenna elements 5 on thereceiver side is chosen on the basis of economical considerations toprovide increased channel diversity; in the case of mobile telephony, asingle base station serves many hundreds or even thousands of mobileunits and it is therefore more economical to add antenna elements to thebase station than to the mobile units. In the case of a local areanetwork (‘LAN’), for example, the cost of the remote stations is lesscritical and a higher number of antennas will be chosen on the receiverside.

[0024] Each transmit antenna element 1 to 4 transmits over a variety ofpaths to each of the receive antenna elements 5. Thus, considering them^(th) receive antenna element out of a total of M, each of the transmitantenna elements 1 to 4 transmits to the receive antenna element m overa variety of paths due to multiple reflections and scattering, whichintroduce complex multi-path fading; however, for simplification, theprocessing of the signals at the receiver is described and illustratedas if they were subject to flat fading (equivalent to transmission overa single path with no inter-path interference) that can be representedby a complex channel transfer coefficient h_(1m) to h_(4m).

[0025] In operation, symbols s₁, s₂, s₃, s₄ are derived from the data tobe transmitted and applied to the transmit antennas 1, 2, 3 and 4. Thereceiver side of the system comprises a detector 6 which receivessignals from the receive antenna element array 5 and detects the symbolss₁ to s₄ from the receive antenna elements.

[0026] On the transmit side of the system, a channel state informationunit extracts weights w₁, w₂, w₃ and w₄ that are, in general terms, acomplex function of the channel transfer coefficients h₁, h₂, h₃, and h₄for each of the transmit antenna elements 1, 2, 3, 4. Beforetransmission, the signal to be transmitted from each of the antennaelements 1 to 4 is multiplied by the respective weight w₁ to w₄. Theweight is again a complex coefficient, which is a function of thetransfer channel coefficient and hence the signal may be modified inphase and/or amplitude as a function of the channel state information.

[0027] The data symbols to be transmitted are encoded in symbol blocksand the symbols are permuted over time within each block between thetransmit antenna elements 1 to 4 with respective replications andcomplex conjugations and/or negations, so that the received signal isdetectable at the receiver side using an orthogonal detection matrixscheme. The encoding scheme matrix for the symbol blocks is shown at 8in FIG. 1.

[0028] The symbols s₁ to s₄ are permuted over the transmit antennaelements a number of times which is a power of 2, the power beinggreater than or equal to 2, the block comprising four permutations inthe present case. It is also possible for the transmitter to includethree antenna elements, the block of symbols preferably comprising fourpermutations in this case also. A higher number of permutations may alsobe utilised but will prolong the symbol block transmission.

[0029] As shown in FIG. 1, the symbols within each block are per mutedin pairs within respective subsets of the symbols and transmitted overcorresponding subsets of the transmit antenna elements 1 to 4, thesubsets of symbols subsequently being permuted between the subsets ofthe transmit antenna elements. Thus, as shown in FIG. 1, symbols s₁, s₂are transmitted initially over transmit antenna elements 1, 2 andsymbols s₃, s₄ are transmitted initially over transmit antenna elements3, 4. In the next step, the negation and conjugation of the symbol s₂ istransmitted over the transmit antenna element 1 and the conjugation ofthe symbol s₁ is transmitted over the transmit antenna element 2, thenegation and conjugation of the symbol s₄ being transmitted over thetransmit antenna element 3 and the conjugation of the symbol s₃ beingtransmitted over the transmit antenna element 4. It will be understoodthat the symbols s₁ and s₂ and their negations and/or their conjugationsconstitute a first subset of symbols that is transmitted over the subsetof transmit antenna elements 1 and 2 with permutations and the symbolss₃ and s₄ with their negations and/or conjugations are transmitted overthe subset of transmit antenna elements 3 and 4 with permutations. Inthe next step, the subset including symbols s₃ and s₄ is transmittedover the subset of transmit antenna elements 1 and 2 with permutationswhile the subset of symbols s₁, s₂ is transmitted over the subset oftransmit antenna elements 3 and 4 with permutations.

[0030] This encoding scheme is a scheme of the kind ABBA. Thisembodiment of the present invention enables this encoding scheme to bedecoded by an orthogonal detection matrix scheme at the receiver. It isalso possible for other encoding schemes of analogous nature to bedecoded using an orthogonal detection matrix scheme, for instanceABB*-A*, or AB-B*A*. Moreover, the space-time code has an overall codingrate of one (that is to say that the data rate is as high as in a singleantenna case) and the system derives full benefit from the spatialdiversity of the multiple transmit and receive antenna elements at thetransmitter and receiver. The fact that the detection scheme uses anorthogonal matrix enables the detection to be performed with lowcomputational cost. Interference terms that would be present with anon-orthogonal detection matrix scheme are substantially cancelled outby the application of the weights w₁ to w₄ to the signals transmitted asa function of the estimated channel transfer functions.

[0031] The signal Y_(m) received by the m^(th) antenna over four timeinstants within the symbol block can be written as $\begin{matrix}{\underset{\underset{Y_{m}}{}}{\begin{bmatrix}\begin{matrix}\begin{matrix}y_{m,1} \\y_{m,2}^{*}\end{matrix} \\y_{m,3}\end{matrix} \\y_{m,4}^{*}\end{bmatrix}} = {{\underset{\underset{H_{m}}{}}{\begin{bmatrix}{h_{1m}w_{1}} & {h_{2m}w_{2}} & {h_{3m}w_{3}} & {h_{4m}w_{4}} \\{h_{2m}^{*}w_{2}^{*}} & {{- h_{1m}^{*}}w_{1}^{*}} & {h_{4m}^{*}w_{4}^{*}} & {{- h_{3m}^{*}}w_{3}^{*}} \\{h_{3m}w_{3}} & {h_{4m}w_{4}} & {h_{1m}w_{1}} & {h_{2m}w_{2}} \\{h_{4m}^{*}w_{4}^{*}} & {{- h_{3m}^{*}}w_{3}^{*}} & {h_{2m}^{*}w_{2}^{*}} & {{- h_{1m}^{*}}w_{1}^{*}}\end{bmatrix}}\quad \underset{\underset{S}{}}{\begin{bmatrix}\begin{matrix}\begin{matrix}s_{1} \\s_{2}\end{matrix} \\s_{3}\end{matrix} \\s_{4}\end{bmatrix}}} + \underset{\underset{B_{m}}{}}{\begin{bmatrix}\begin{matrix}\begin{matrix}b_{m,1} \\b_{m,2}^{*}\end{matrix} \\b_{m,3}\end{matrix} \\b_{m,4}^{*}\end{bmatrix}}}} & {{Equation}\quad 1}\end{matrix}$

[0032] where y_(m,1) to y_(m,4) represent the signals received from thetransmit antenna elements 1 to 4 respectively, H_(m) represents thematrix obtained by multiplying the channel transfer functions by thecorresponding weights applied to the transmit antenna elements, Srepresents the symbols s₁ to s₄ transmitted from the transmit antennaelements 1 to 4 respectively, b_(m,1) to b_(m,4) represent the noise andinterference at the m^(th) receive antenna element and B_(m) representsthe received noise matrix. In this equation, the minus sign representsthe negation of the corresponding value and the asterisk sign representsthe conjugate of the value.

[0033] These multiple received signals are processed at the receiveraccording to the Maximum Ratio Combining technique. That is to say, thereceived pilot signals for each transmit antenna element are measured inorder to estimate the channel transfer coefficients h_(1m) to h_(4m) andthe weights applied at the transmitter side w₁ to w₄ and the Hermitiantransposes Ĥ_(m) ^(H) of the estimated channel transfer coefficientmatrices for each receive antenna element m are calculated. The receivedsymbol blocks Y_(m) are multiplied by the corresponding Hermitiantransposes Ĥ_(m) ^(H) and the resulting multiplied signals are summedover the antennas, and we finally get a new signal Z such that$\begin{matrix}{Z = {{\sum\limits_{m = 1}^{M}{{\hat{H}}_{m}^{H}Y_{m}}} = {{\left( {\sum\limits_{m = 1}^{M}{{\hat{H}}_{m}^{H}H_{m}}} \right)S} + {\sum\limits_{m = 1}^{M}{{\hat{H}}_{m}^{H}{B_{m}.}}}}}} & {{Equation}\quad 2}\end{matrix}$

[0034] Provided that the channel estimation is sufficently accurate andthe weights actually applied to the signals to be transmitted alsocorrespond accurately to the calculated weights, the resulting detectionmatrix $\sum\limits_{m = 1}^{M}{{\hat{H}}_{m}^{H}H_{m}}$

[0035] corresponds with a sufficient degree of approximation to thedetection matrix of the ideal orthogonal rate 1 code scheme for properdetection of the data, that is to say $\begin{matrix}{{{\sum\limits_{m = 1}^{M}{H_{m}^{H}H_{m}}} = {{\begin{bmatrix}A & 0 & 0 & 0 \\0 & A & 0 & 0 \\0 & 0 & A & 0 \\0 & 0 & 0 & A\end{bmatrix}\quad {where}\quad A} = {\sum\limits_{m = 1}^{M}{\sum\limits_{n = 1}^{4}{h_{nm}}^{2}}}}},} & {{Equation}\quad 3}\end{matrix}$

[0036] if the transmit weights satisfy at least approximately thefollowing relation: $\begin{matrix}{{\Re \left\{ {{\sum\limits_{m = 1}^{M}{h_{1m}^{*}h_{3m}w_{1}^{*}w_{3}}} + {\sum\limits_{m = 1}^{M}{h_{2m}^{*}h_{4m}w_{2}^{*}w_{4}}}} \right\}} = 0.} & {{Equation}\quad 4}\end{matrix}$

[0037] where h_(nm) is the channel transfer coefficient of the channelbetween the n^(th) transmit antenna element and the m^(th) receiveantenna element, w_(n) is the weight applied to the signal of the n^(th)transmit antenna element and

represents the real part of the value on which it operates.

[0038] Due to the orthogonality of the code scheme, the detection stepcan then be performed with a low computational cost.

[0039] Several sets of transmit weights may be used to solve thisequation in accordance with this embodiment of the present invention.One example consists in choosing the four weights such that$\begin{matrix}{{w_{1} = 1}{w_{2} = 1}{w_{3} = {\exp \left( {j\left\lbrack {{{angle}\left( {\sum\limits_{m = 1}^{M}{h_{1m}h_{3m}^{*}}} \right)} + {\pi/2}} \right\rbrack} \right)}}{w_{4} = {{\exp \left( {j\left\lbrack {{{angle}\left( {\sum\limits_{m = 1}^{M}{h_{2m}h_{4m}^{*}}} \right)} + {\pi/2}} \right\rbrack} \right)}.}}} & {{Equation}\quad 5}\end{matrix}$

[0040] With this choice of weights, it is sufficient for thetransmitting side to obtain phase information for the weightingoperation on two out of the four transmit antenna elements, whichreduces the amount of feedback information to be transmitted from thereceiver side if the channel state information is measured at thereceiver side, for example.

[0041] The detection scheme for three emitting antennas can easily bederived from this four antenna coding scheme. Four complex symbols aretransmitted from three antennas over four time instants, for instance byturning off the 4^(th) antenna, which corresponds to set h_(4m)=0 in theprevious equations. In this case the overall space-time scheme is anorthogonal one if and only if the transmit weights satisfy:$\begin{matrix}{{{\Re \left\{ {\sum\limits_{m = 1}^{M}{h_{1m}^{*}h_{3m}w_{1}^{*}w_{3}}} \right\}} = 0},} & {{Equation}\quad 6}\end{matrix}$

[0042] for instance by choosing $\begin{matrix}{w_{1} = {w_{2} = {{1\quad {and}\quad w_{3}} = {{\exp \left( {j\left\lbrack {{{angle}\left( {\sum\limits_{m = 1}^{M}{h_{1m}h_{3m}^{*}}} \right)} + {\pi/2}} \right\rbrack} \right)}.}}}} & {{Equation}\quad 7}\end{matrix}$

[0043] With this choice of weights, it is sufficient for thetransmitting side to obtain phase information the weighting operation onone only out of the three transmit antenna elements.

[0044] The above conditions for the weighting scheme to enable decodingby an orthogonal detection matrix are applicable to an encoding schemeof the kind ‘ABBA’, that is to say where sub-sets A and B of symbols s₁,s₂ and s₃, s₄ and conjugated symbols s₁*, s₂* and s₃*, s₄* and/ornegated conjugated symbols −s₁*, −s₂* and −s₃*, −s₄* symbols arepermuted over time between sub-sets 1, 2 and 3, 4 of antenna elementswithout negation nor conjugation of the symbol sub-sets, according tothe matrix $\begin{bmatrix}A & B \\B & A\end{bmatrix}.$

[0045] Other encoding schemes may be utilised of the form$\begin{bmatrix}A & B \\B^{*} & {- A^{*}}\end{bmatrix}\quad {{or}\quad\begin{bmatrix}A & B \\{- B^{*}} & A^{*}\end{bmatrix}}\quad {{{or}\quad\begin{bmatrix}b_{1} & b_{2}^{*} & b_{3} & b_{4}^{*} \\b_{2} & {- b_{1}^{*}} & {- b_{4}} & b_{3}^{*} \\b_{3} & b_{4}^{*} & {- b_{1}} & {- b_{2}^{*}} \\b_{4} & {- b_{3}^{*}} & b_{2} & {- b_{1}^{*}}\end{bmatrix}}.}$

[0046] In the absence of weighting before transmission, these encodingschemes would leave interference terms that would require a detectionmatrix $\sum\limits_{m = 1}^{M}{H_{m}^{H}H_{m}}$

[0047] that is non-orthogonal.

[0048] In order to be able to detect the signals using an orthogonaldetection matrix, in accordance with another embodiment of the presentinvention the weightings applied to the signals to be transmitted arederived such that $\begin{matrix}{{\Re \left\{ {{\sum\limits_{m = 1}^{M}{h_{pm}^{*}w_{p}^{*}h_{rm}w_{r}}} \pm {\sum\limits_{m = 1}^{M}{h_{qm}^{*}w_{q}^{*}h_{sm}w_{s}}}} \right\}} = 0.} & {{Equation}\quad 8}\end{matrix}$

[0049] In a preferred realisation of this embodiment, $\begin{matrix}\left\{ \begin{matrix}{w_{p} = {w_{q} = 1}} \\{w_{r} = {\exp \left( {j \times \left\lbrack {{{angle}\left\{ {\sum\limits_{m = 1}^{M}{h_{pm}h_{rm}^{*}}} \right\}} + {\pi/2}} \right\rbrack} \right)}} \\{w_{s} = {\exp \left( {j \times \left\lbrack {{{angle}\left\{ {\sum\limits_{m = 1}^{M}{h_{qm}h_{sm}^{*}}} \right\}} + {\pi/2}} \right\rbrack} \right)}}\end{matrix} \right. & {{Equation}\quad 9}\end{matrix}$

[0050] In accordance with yet other embodiments of the presentinvention, in equation 8, $\begin{matrix}{{\left\{ {{\sum\limits_{m = 1}^{M}{h_{pm}^{*}w_{p}^{*}h_{rm}w_{r}}} \pm {\sum\limits_{m = 1}^{M}{h_{qm}^{*}w_{q}^{*}h_{sm}w_{s}}}} \right\}} = 0} & {{Equation}\quad 10}\end{matrix}$

[0051] where ℑ represents the imaginary part of the value it operatesand the scheme is decodable using an orthogonal detection matrix.

[0052] Preferably, $\begin{matrix}\left\{ \begin{matrix}{w_{p} = {w_{q} = 1}} \\{w_{r} = {\exp \left( {j \times {angle}\left\{ {\sum\limits_{m = 1}^{M}{h_{pm}h_{rm}^{*}}} \right\}} \right)}} \\{w_{s} = {\exp \left( {j \times {angle}\left\{ {\sum\limits_{m = 1}^{M}{h_{qm}h_{sm}^{*}}} \right\}} \right)}}\end{matrix} \right. & {{Equation}\quad 11}\end{matrix}$

[0053]FIG. 2 shows the application of a system in accordance with FIG. 1to a time division duplex (TDD) system based on orthogonal frequencydivision multiplexing (OFDM) modulation, as specified for example in theHiperlan/2 standard of the ETSI but modified to include the space-timetransmit diversity system of the embodiment of the present inventionshown in FIG. 1. The embodiment of the invention shown in FIG. 2 hasfour transmit aerials at the base station, but it is also possible forthe base station to have three transmit aerials. The base station of thesystem is shown at 13 and one of a number of subscriber units is shownat 14.

[0054] At the base station 13, data is input to the encoder 15 where itis encoded to include error correction information. The resulting datatrain is supplied to a data-mapping and block coding unit that forms thedata train into symbols and performs the negation and conjugationoperations to produce the symbol blocks according to the encoding scheme12. The data from the mapping and block coding unit 16 is supplied tothe multipliers 8, 9, 10 and 11 where, for each sub-carrier of frequencyf, it is multiplied by respective complex weighting coefficients w_(1f)to w_(4f) and applied to respective elements of an array of OFDMmodulation units that feed the transmit antennas 1 to 4. It is alsopossible to apply interleaving of the data after the encoder 15.

[0055] Pilot signals are included in the transmitted signals for eachtransmit antenna element, without weighting, to enable estimation of thedownlink channels. A permutation signal is also added that is indicativeof the number of permutations in a symbol block. For example, especiallyduring deployment of the present invention, there may be a mix of basestations with only two transmit antenna elements and therefore twopermutations per symbol block and base stations in accordance with thepresent invention with more than two transmit antenna elements andtherefore more than two permutations per symbol block. The permutationsignal takes a distinctive value, at least in the latter case, to enablethe receiver to adapt the number of permutations performed in detectingthe signal to the number made in the transmitted signal.

[0056] The channel state information is calculated in the calculator 7at the base station from a similar pilot signal included in the uplinktransmissions from the respective subscriber unit 14 and received at thebase station over the antenna elements 1, 2, 3 and 4 and detected by thereceiver unit 14 of the base station; since the system is a timedivision duplex system with the same carrier frequencies used for thedownlink and the uplink, the measurements made on the uplink pilot areconsidered to be a sufficient approximation to the state of the downlinksignal.

[0057] At the subscriber unit 14, the transmitted signals are receivedover the array of receive antenna elements 5, which are also used fortransmission of signals back to the base station. The received signalsare demodulated in respective OFDM channel demodulators 18. The channeltransfer coefficients are estimated by an array of channel estimators 19from the downlink pilot signal and applied to respective receiverelements of an array 20, together with the permutation signal thatindicates the number of permutations to be performed in detecting thesymbols. The receiver elements of array 20 calculates the Hermitiantransform Ĥ_(m) ^(H) of the channel transfer matrix, which it uses tomultiply the received signals in the array of receivers 20.

[0058] The processed signals from the array of receivers 20 are appliedto the maximum ratio combination summer 21 that adds the signals fromthe receiver array 20 over the antenna elements 5. Because of thetransmit weights w₁ to w₄ applied at the transmitter, the detectionmatrix scheme is an orthogonal matrix. The signal from the maximum ratiocombiner 21 is passed to a matrix computation unit 22 that recovers thedigital signal train from the symbol blocks. The digital signal train ispassed to a decoder 23 that applies the error correction process andrecovers the data.

[0059] The system described with reference to FIG. 2 is a time divisionduplex system utilising orthogonal frequency division multiplexing. Thisenables the weighting of the transmit channels to be calculated at thebase station using measurement of a pilot signal in the uplink signaltransmitted from the subscriber station as an approximation for thechannel state information of the downlink signal transmitted from thebase station. Since the same antenna elements both at the base stationand at the subscriber station are used for reception and transmission,this approximation is valid, and, indeed, the approximation is alsovalid in certain circumstances even where the antenna elements used fortransmission and reception are not identical for the uplink anddownlink.

[0060] The system shown in FIG. 3 is a frequency division duplex systembased on the CDMA (code division multiple access) standards with amodification to provide some feedback information from the mobile unitsto the base station to provide some channel state information concerningthe downlink signal. Such a system is compatible with the 3GPP or 3GPP2standard if an adaptation to the standard were introduced to accommodatethe channel state information fed back from the mobile unit to the basestation.

[0061] Referring now to FIG. 3 in more detail, the base stationcomprises a transmitter part shown generally at 24 and a receiver part25. Input data, together with a pilot signal and a permutation signalindicative of the number of permutations made during the space-timetransmit diversity permutations is applied to the encoder 15. Theencoder 15 includes error correction data and the resulting signal isapplied to the data mapping and block coding unit 16 that assembles thetrain of digital data into symbol blocks with permutations, negationsand conjugations according to the encoding scheme. Multipliers 8 to 11then multiply the data signals by respective weights for the respectiveantenna elements 1, 2, 3 and 4. In accordance with the CDMAspecifications, the signals are spread over different frequencysub-carrier bands before transmission by an array of spreaders 26.

[0062] In the present embodiment of the invention, the channel stateinformation is calculated at the mobile unit and transmitted back to thebase station on the uplink over the same antenna elements as used forthe downlink. In the preferred embodiment, the parameters as inequations 9 or 10 for the weights w₁ to w₄ to be applied to themultipliers 8 to 11 are calculated at the mobile unit and transmitted onthe uplink to the base station, as this reduces the amount of feedbackinformation passing over the communication link, being only a phaseinformation for the weights w₃ and w₄ (w₃ only in the case of threetransmit antenna elements), the weights w₁ and w₂ being constant values.The signals received at the base station antenna elements 1 to 4 aredecoded in the receiver part 25 of the base station. The weightinginformation signal is extracted by a detector 27 and supplied to thechannel state calculator 7 to calculate the weights applied to themultipliers 8 to 11.

[0063] The mobile unit comprises a receiver part indicated generally at28 and a transmitter part 29. At the mobile unit, the signalstransmitted are received on the antenna element array 5 and applied to acorresponding array of despreaders 30 that supply the base band signalsto the array of receiver channel transfer function estimators at themobile unit and to an array of receivers 31, each “finger” or signalreceived over a different transmission path being detected separatelyand the fingers being reassembled. The channel state information iscalculated from a pilot signal transmitted by the base station. Thechannel state information is supplied on one hand to the mobile unittransmitter part 29 for retransmission to the base station in the uplinksignal (which is at a different frequency from the downlink signal) andto the array of receiver elements 31.

[0064] The receiver elements multiply the signals from the despreaderarray 30 by the coefficients of the Hermitian transform Ĥ_(m) ^(H) ofthe channel transfer matrix obtained by permutation, transpositionnegation and conjugation operations on the channel state informationsignals, the number of permutations being defined by the receivedpermutation signal. The resulting signals from each receiver antennaelement are then summed over all fingers in an array of maximum ratiocombiners 32 and further summed in a maximum ratio combiner 21 over thedifferent antenna elements. Once again, the application of the transmitweights corresponding to equation 4 ensures that the detection matrix isan orthogonal matrix that enables the calculations to be greatlysimplified. The symbols from the maximum ratio combiner 21 are appliedto the matrix computation unit 22 and converted to a chain of digitalsignals and the decoder 23 detects and recovers the data with errordetection.

[0065] In a preferred embodiment of this type of system, the transmitterpart 29 of the mobile unit is similar to the transmitter part 24 of thebase station in operation, and the receiver part 25 of the base stationis similar to the receiver part 28 of the mobile unit. Adaptations areof course made to the number of antenna elements in the array 5 at thesubscriber station. In this way, advantage is taken of the space-timetransmit diversity performance of the present invention on the uplinkfrom the subscriber unit to the base station as well as on the downlinkfrom the base station to the mobile unit.

[0066] In another embodiment of the invention, the number of antennaelements applied at the mobile unit is reduced, for example to twoantenna elements, with a view to reducing to the cost of the mobileunit. In this case, the spatial diversity is, of course, reducedcompared to a system with four transmit antennas in the array 5.

1. A method of transmitting data from a transmitter to a remote receiverusing transmit diversity wireless communication, said transmittercomprising at least three transmit antenna elements, the methodcomprising modifying signals to be transmitted over at least one of saidtransmit antenna elements as a function of channel information at leastapproximately related to the channel transfer function of thetransmitted signals (h₁, h₂, h₃, and h₄), encoding said data in symbolblocks, and at least one of said sub-sets of said transmit antennaelements comprising more than one of said transmit antenna elements,characterised in that pairs of the symbols (s₁, s₂, s₃, s₄) of a blockbeing permuted within respective sub-sets of symbols and permutedsymbols between the transmit antenna elements of respective sub-sets ofsaid transmit antenna elements over time with respective replicationsand complex conjugations and/or negations, and said sub-sets of symbolsand permuted symbols are permuted over time between said sub-sets oftransmit antenna elements so that the received signal being detected atthe receiver by a detection scheme using an orthogonal detection codematrix.
 2. A method as claimed in claim 1, characterised in that saidsignals transmitted over said transmit antenna elements are modified inphase as a function of said channel information.
 3. A method as claimedin claim 1, characterised in that said channel information is a functionof signals received at said receiver from said transmitter and istransmitted from said receiver to said transmitter.
 4. A method asclaimed in claim 1, wherein said receiver additionally comprisestransmitting means for transmitting signals over channels similar to thetransmit channels of said transmitter, and said transmitter additionallycomprises receiving means (14, 25) including said transmit antennaelements for receiving signals transmitted from said transmitting means,characterised in that said channel information is calculated at saidtransmitter as a function of the channel transfer function of signalstransmitted from said transmitting means to said receiving means.
 5. Amethod as claimed in claim 1, characterised in that said symbols (s₁,s₂, s₃, s₄) are permuted over time within said block between thetransmit antenna elements a number of times equal to a power of two,which power is greater than or equal to two.
 6. A method as claimed inclaim 4, characterised in that said transmitter comprises three or fourof said transmit antenna elements and said symbols (s₁, s₂, s₃, s₄) arepermuted over time within said block four times between the transmitantenna elements.
 7. A method as claimed in claim 1, characterised inthat said sub-sets of symbols and permuted symbols are permuted overtime between said sub-sets of transmit antenna elements withoutconjugation or negation and said signals transmitted over at least oneof said transmit antenna elements are modified so as to satisfy at leastapproximately the equation${\Re \left\{ {{\sum\limits_{m = 1}^{M}{h_{1m}^{*}h_{3m}w_{1}^{*}w_{3}}} + {\sum\limits_{m = 1}^{M}{h_{2m}^{*}h_{4m}w_{2}^{*}w_{4}}}} \right\}} = 0$

where the complex numbers h_(1m), h_(2m), h_(3m) and h_(4m) representthe actual channel transfer functions over the transmit antenna elementsand the m^(th) receiver transmit antenna element, the complex numbersw₁, w₂, w₃ and w₄ represent the modifications applied to the signals atthe transmit antenna elements, x* represents the complex conjugate ofthe number x and the operator

represents the real part of a complex value.
 8. A method as claimed inclaim 1, characterised in that said signals transmitted over at leastone of said transmit antenna elements are modified so as to satisfy atleast approximately the equation${\Re \left\{ {{\sum\limits_{m = 1}^{M}{h_{pm}^{*}w_{p}^{*}h_{rm}w_{r}}} \pm {\sum\limits_{m = 1}^{M}{h_{qm}^{*}w_{q}^{*}h_{sm}w_{s}}}} \right\}} = 0$

or the equation${\left\{ {{\sum\limits_{m = 1}^{M}{h_{pm}^{*}w_{p}^{*}h_{rm}w_{r}}} \pm {\sum\limits_{m = 1}^{M}{h_{qm}^{*}w_{q}^{*}h_{sm}w_{s}}}} \right\}} = 0$

where the complex number h_(n,m) represents the actual channel transferfunction over the nth transmit antenna element and the mth receivertransmit antenna element, the complex number w_(n) represents themodification applied to the signals at the nth transmit antenna element,x* represents the complex conjugate of the number x,

represents the real part of a complex value and ℑ represents theimaginary part of a complex value.
 9. A method as claimed in claim 1,characterised in that said transmitter comprises four of said transmitantenna elements and said signals transmitted over said transmit antennaelements are modified so as to satisfy at least approximately theequations $\begin{matrix}{w_{1} = 1} \\{w_{2} = 1} \\{w_{3} = {\exp \left( {j\left\lbrack {{{angle}\left( {\sum\limits_{m = 1}^{M}{{\hat{h}}_{1m}{\hat{h}}_{3m}^{*}}} \right)} + {\pi/2}} \right\rbrack} \right)}} \\{w_{4} = {\exp \left( {j\left\lbrack {{{angle}\left( {\sum\limits_{m = 1}^{M}{{\hat{h}}_{2m}{\hat{h}}_{4m}^{*}}} \right)} + {\pi/2}} \right\rbrack} \right)}}\end{matrix}$

where the complex number ĥ_(n,m) represents the measured channeltransfer function over the nth transmit antenna element and the mthreceiver antenna element, M represents the total number of receiverantenna elements at said receiver, the complex number w_(n) representsthe modification applied to the signals at the nth transmit antennaelement and x* represents the complex conjugate of the number x.
 10. Amethod as claimed in claim 9, characterised in that values at leastapproximately related to w₃ and w₄ are calculated at said receiver as afunction of received signals and transmitted from said receiver to saidtransmitter.
 11. A method as claimed in claim 9, wherein said receiveradditionally comprises transmitting means for transmitting signals overchannels similar to the transmit channels of said transmitter, and saidtransmitter additionally comprises receiving means including saidtransmit antenna elements for receiving signals transmitted from saidtransmitting means, characterised in that values at least approximatelyrelated to w₃ and w₄ are calculated at said transmitter as a function ofthe channel transfer function (h₁, h₂, h₃, and h₄) of signalstransmitted from said transmitting means to said receiving means.
 12. Amethod as claimed in claim 1, characterised in that said transmittercomprises three of said transmit antenna elements and said signalstransmitted over said transmit antenna elements are modified so as tosatisfy at least approximately the equations${w_{1} = {w_{2} = 1}},{{{and}\quad w_{3}} = {\exp \left( {j\left\lbrack {{{angle}\left( {\sum\limits_{m = 1}^{M}{{\hat{h}}_{1m}{\hat{h}}_{3m}^{*}}} \right)} + {\pi/2}} \right\rbrack} \right)}}$

where the complex number ĥ_(n,m) represents the measured channeltransfer function over the nth transmit antenna element and the mthreceiver antenna element, M represents the total number of receiverantenna elements at said receiver, the complex number w_(n) representsthe modification applied to the signals at the nth transmit antennaelement and x* represents the complex conjugate of the number x.
 13. Amethod as claimed in claim 12, characterised in that a value at leastapproximately related to w₃ is calculated at said receiver as a functionof received signals and transmitted from said receiver to saidtransmitter.
 14. A method as claimed in claim 12, wherein said receiveradditionally comprises transmitting means for transmitting signals overchannels similar to the transmit channels of said transmitter, and saidtransmitter additionally comprises receiving means (14, 25) includingsaid transmit antenna elements for receiving signals transmitted fromsaid transmitting means, characterised in that a value at leastapproximately related to w₃ is calculated at said transmitter as afunction of the channel transfer function (h₁, h₂, h₃, and h₄) ofsignals transmitted from said transmitting means to said receivingmeans.
 15. A method as claimed in claim 1, characterised in that apermutation signal indicative of the number of permutations over time ofsaid symbols (s₁, s₂, s₃, s₄) within said block is transmitted from saidtransmitter to said receiver and said receiver is responsive to saidpermutation signal in the numbers of permutations it performs indetecting the data transmitted.
 16. A system for transmitting data by atransmit diversity wireless communication method as claimed in claim 1,the system comprising said transmitter and a plurality of said remotereceivers, said transmitter comprising at least three of said transmitantenna elements and transmit encoding means for modifying signalstransmitted over at least one of said transmit antenna elements as afunction of channel information at least approximately related to thechannel transfer function (h₁, h₂, h₃, and h₄) of the transmittedsignals and for encoding said data in symbol blocks (12), and at leastone of said sub-sets of said transmit antenna elements comprising morethan one of said transmit antenna elements, characterised in that pairsof the symbols of a block being permuted within respective sub-sets ofsymbols and Permuted symbols between the transmit antenna elements ofrespective sub-sets of said transmit antenna elements over time withrespective replications and complex coniugations and/or negations, andsaid transmit encoding means is arranged to permute said sub-sets ofsymbols and permuted symbols over time between said sub-sets of transmitantenna elements so that the received signal is detected at the receiverby a detection scheme using an orthogonal detection code matrix.
 17. Asystem as claimed in claim 16, characterised in that said receivercomprises means for calculating said channel information as a functionof pilot signals received from said transmitter and for transmittingsaid channel information from said receiver to said transmitter.
 18. Asystem as claimed in claim 16, wherein said receiver additionallycomprises transmitting means for transmitting signals over channelssimilar to the transmit channels of said transmitter, and saidtransmitter additionally comprises receiving means including saidtransmit antenna elements for receiving signals transmitted from saidtransmitting means, characterised in that said transmit encoding meansat said transmitter is responsive to the channel transfer function ofpilot signals transmitted from said transmitting means to said receivingmeans to calculate said channel information.
 19. A system as claimed inclaims 16, characterised in that said encoding means is arranged totransmit a permutation signal indicative of the number of permutationsover time of said symbols within said block and said receiver comprisesdetection means responsive to said permutation signal in the number ofpermutations it performs in detecting the data transmitted.
 20. Atransmitter for transmitting data to a remote receiver by a transmitdiversity wireless communication method as claimed in claim 1,comprising at least three of said transmit antenna elements and transmitencoding means for modifying signals transmitted over at least one ofsaid transmit antenna elements as a function of channel information atleast approximately related to the channel transfer function (h₁, h₂,h₃, and h₄) of the transmitted signals and for encoding said data insymbol blocks and at least one of said sub-sets of said transmit antennaelements comprising more than one of said transmit antenna elements,characterised in that pairs of the symbols of a block being permutedwithin respective sub-sets of symbols and permuted symbols between thetransmit antenna elements of respective sub-sets of said transmitantenna elements over time with respective replications and complexconjugations and/or negations, and said transmit encoding means isarranged to permute said sub-sets of symbols and permuted symbols overtime between said sub-sets of transmit antenna elements, so that thereceived signal is detected at the receiver by a detection scheme usingan orthogonal detection code matrix.
 21. A transmitter as claimed inclaim 20, for transmitting data to a receiver additionally comprisingtransmitting means for transmitting signals over channels similar to thetransmit channels of said transmitter, characterised in that saidtransmitter additionally comprises receiving means including saidtransmit antenna elements for receiving signals transmitted from saidtransmitting means, said transmit encoding means at said transmitterbeing responsive to the channel transfer function (h₁, h₂, h₃, and h₄)of signals transmitted from said transmitting means to said receivingmeans to calculate said channel information.
 22. A transmitter asclaimed in claim 20, characterised in that said encoding means isarranged to transmit a permutation signal indicative of the number ofpermutations over time of said symbols within said block.
 23. A receiverfor receiving data transmitted by a transmit diversity wirelesscommunication method as claimed in claim 1 from a transmitter comprisingat least three of said transmit antenna elements, said receivercomprises detection means for detecting data encoded in symbol blocks,and said signals transmitted over at least one of said transmit antennaelements having been modified as a function of channel information atleast approximately related to the channel transfer function (h₁, h₂,h₃, and h₄) of the transmitted signals characterised in that saiddetection means is arranged to detect sub-sets of symbols and permutedsymbols, the symbols (s₁, s₂, s₃, s₄) of a block having been permutedwithin respective sub-sets of symbols and Permuted symbols between thetransmit antenna elements of respective sub-sets of said transmitantenna elements over time with respective replications and complexconjugations and/or negations and permuted over time between saidsub-sets of transmit antenna elements so that the received signal isdetected at the receiver by a detection scheme using an orthogonaldetection code matrix.
 24. A receiver as claimed in claim 23,characterised in that said detection means at said receiver comprisesmeans for calculating said channel information as a function of signalsreceived from said transmitter and for transmitting said channelinformation from said receiver to said transmitter.
 25. A receiver asclaimed in claim 23, characterised in that said detection means isresponsive to a permutation signal indicative of the number ofpermutations over time of said symbols within said block, and which istransmitted from said transmitter to said receiver, in the numbers ofpermutations it performs in detecting the data transmitted.